Known methods of introducing additives to polymeric particles include dry blending the materials, melting, and compounding the melted blend with extruders and pelletizing or powdering to the desired physical form. The additives employed to treat polymeric particles include antioxidants, processing aids, slip agents, antiblocking agents, antistatic agents, lubricants, UV stabilizers, coupling agents and colorants.
Another method of introducing additives to polymeric particles is to contact such particles with additive at the extruder hopper during end use processing. Additives such as colorants, slip agents, processing aids, blowing agents, and others are introduced to virgin polymeric particles at this stage usually in concentrate form. In many instances, difficulty is encountered in metering the exact amounts of additive concentrate necessary to do a specific job. This is especially true for additives such as processing aids and external lubricants which are used at very low levels and usually cannot be added in a concentrate form.
Some polymers are presently being manufactured with technology that does not lend itself to such techniques as melt compounding and pelletizing. Many polymers such as high density polyethylene, linear low density polyethylene, and polypropylene emerge from the polymerization reactor in a dry granular form, i.e., in a form similar to that of a fluidized bed system. Presently, additives for these polymers must be introduced by melting, compounding, and then pelletizing. This extra step increases the cost of such polymer manufacturing operations and can adversely affect the properties of such polymers.
Another method of coating polymeric particles with additives is disclosed in Japanese Patent 56-021815, issued to Tokoyama Soda. This patent teaches contacting polypropylene granules with a dispersion of additives in a solvent, followed by removal of the solvent. While some stabilization is thereby imparted to the coated polypropylene granules, the treated pellets have severe static electricity problems when subsequently processed, such as for example, by extrusion.
It would be desirable to have available a simple process for applying additives to polymeric particles in order to enhance the stability of the particles without adversely affecting the physical properties and processability of the treated particles. Typical additives employed for treating polymeric particles have been applied using organic solvents since such additives are typically insoluble in water. Due to the ever increasing cost of organic solvents, the high cost of solvent recovery systems, as well as the toxicity and explosive nature of organic solvents, plus the strict air-quality controls imposed by federal regulations, there is a great deal of motivation for users of polymer additives to switch to water as the solvent of choice for applying such additives to polymeric particles.
Yet another prior art process for the application of additives to polymeric particles involves the preparation of aqueous emulsions of additives, as disclosed, for example, in Patent Cooperation Treaty Document WO 86/04909. This process also discloses coating of polymeric particles by employing an aqueous emulsion or dispersion of additives, such as antioxidants, thermal stabilizing agents, colorants, and the like. These aqueous emulsions or dispersions can be applied by spray, wipe, dip systems, or the like to coat the polymeric particles before their normal drying or degassing operations.
While the latter application system represents an advance in the state of the art by replacing the need for organic solvents with aqueous-based application systems, the main disadvantage of this aqueous application system is that it contains substantial quantities of water, which requires special handling and shipping of the aqueous emulsions or dispersions. For example, these emulsions or dispersions tend to freeze when exposed to extremely cold temperatures. Unfortunately, merely heating the frozen emulsion or suspension does not always result in the re-formation of stable emulsions or dispersions. Thus, exposure to temperature extremes and long term storage can cause problems in processing these aqueous emulsions or dispersions when it is attempted to apply these additive systems to polymer particles.
Copending application Ser. No. 392,759, filed Aug. 11, 1989 now U.S. Pat. No. 5,204,022, discloses a process for preparation of water-dispersible additives in solid form wherein a grinding step is required to achieve the additives in solid powder form. If the melting temperature of the additives is less than 100.degree. C., cryogenic grinding is needed to obtain additives in powder form. The cryogenic grinding is costly. Therefore, it would be desirable to develop a process which eliminates the need for cryogenic grinding of the additives with melting temperatures less than 100.degree. C. in order to obtain additives in fine particle form.
In order to overcome the limitations cited above, it would be desirable to prepare additive systems for the treatment of polymer particles wherein the additive systems can be handled in solid form, are readily water dispersible to produce aqueous application systems useful for the treatment of polymer particles, and eliminate the need for cryogenic grinding.